1. Field of the Invention
The present invention relates to current spreading in semiconductor devices, and more particularly to light emitting diodes (LED) having a current spreading layer to improve current distribution in a light emitting region of the device.
2. Background Information
FIGS. 1 and 2 is a schematically illustration of a known LEDs 100, 110. The LED comprises a layer of p-type semiconductor material 102, a layer of n-type semiconductor material 103 and an active, or light generating, layer 104 at the p-n junction. The LED is formed on a substantially transparent substrate of a material 105 such as Sapphire. Terminals 106, 107 are connected to the p-type and n-type semiconductor materials 102, 103 for supplying power to the LED. A small part of the p-type material 102 and p-n junction 104 are removed by etching to expose the n-type material 103 for connection of the n-terminal 107.
When a voltage is applied to terminals 106 and 107 a current flows through the semiconductor materials 102 and 103. Electrons and holes flow into the p-n junction and when an electron meets a hole it falls into a lower energy level which releases a photon, the fundamental element of light. For maximum efficient and light output current must flow generally uniformly through the whole active region. Gallium nitride (GaN) is a popular LED semiconductor material because of its wide bandgap. However, is the difficulty to achieve good ohmic contact between a P-type GaN semiconductor material, for example layer 102, and the electrode terminal 106. If good ohmic contact is not achieved then current tends to crowd and flow only through the area of the active region directly below the terminal. Mush of the light output from this area of the active region is blocked by the terminal itself.
Various techniques are available to solve this current crowding problem. U.S. Pat. No. 5,008,718 describes a popular technique which involves inserting a highly conductive transparent contact layer 108 between the p-type semiconductor layer 102 and terminal 106. The transparent window layer is typically a semiconductor material having a lower electrical resistivity and greater bandgap than the p-type semiconductor layer 102. The drawbacks of this technique are that the transparent window layer is extremely thick, which increases manufacturing cost, and has poor transparency which blocks or absorbs some light.
Another popular layer if to insert a thin film of Indium Tin Oxide (ITO) layer 109 between the terminal 106 and p-type semiconductor layer 102. ITO has a higher transparency than a transparent semiconductor layer, but has poorer current spreading ability due to its single layer structure.
These and other conventional solutions to the current spreading problem often trade of efficiencies between good ohmic contact and transparency o the contact layer. Accordingly, is an object of the present invention to provide a LED device which overcomes at least ameliorates some of the deficiencies exhibited by prior LEDs.